Re: how energy is released from ATP&what is the role of (P) ?

The form in which energy is released when ATP is broken down depends on the 
particular reaction involved. For instance, the hydrolysis of ATP in 
solution to give ADP and phosphate only results in the formation of heat. 
However, most reactions involving the breakdown of ATP in living organisms 
couple the energy released from the hydrolysis of ATP to another chemical 
reaction, in this way the energy is trapped as chemical potential energy. 
Typically, this involves the coversion of a "low energy" substrate to a 
higher enery substrate (by converting it to a phosphorylated compound) such 
that the overall reaction becomes energetically favorable (i.e. it releases 
energy (exothermic) instead of requiring enery (endothermic)). The 
hydrolysis of ATP is also coupled to the active transport of ions and 
molecules across membranes, and the performance of mechanical work (in 
muscle).

The importance of the phosphorous atom is to provide the core of the 
phosphate group that is the basis of the phosphoanhydride bond that has a 
relatively high stndard free energy of hydrolysis. This provides a useful 
biochemical "currency" since the hydrolysis reaction is chemically very 
simple (only requiring water), but containing plenty of energy. In this 
respect it is very different to carbon-carbon bonds, which may contain 
plenty of energy, but are chemically very hard to make and break.

Oxygen is a special atom in energy production as it is a powerful oxidant 
and as it accepts electrons to form water, the energy released bu its 
reduction can be coupled to the formation of many molecules of ATP.

Although the phosphorous atom in ATP cannot be replaced by other elements, 
high energy bonds employing other elements are used in biochemistry. For 
instance, the hydrolysis of acetyl coenzyme A to acetate and free CoA 
releases a similar amount of energy as the hydrolysis of ATP. In this case 
the high energy bond os a carbon-sulfur bond.